how do i fix my original vodafone sony ecricsson standard 512mb memorycard?

how do i fix my standard sony ericsson w810i 512mb memory card?, it was workin perfectly since i bought it brand new form vodafone, but ever since i disconnected it from my computer last night i have run into the following problem- everytime i switch the phone on as normal (after i enter my pin and lock passwords) with my sim and memory card in the phone responds super slow and freezes then the screen begins to flash black and white.

what have i done to my phone/ memorycard and how do i fix/ solve the problem?